The long-term goal of this research is to further understand the interaction between cervical somatosensory and vestibular information in the detection of body position in space and in generating postural responses. Information from the cervical afferents, visual, somatosensory and vestibular systems are integrated to form a three dimensional representation of the head and trunk in space for the generation of appropriate balance responses. Previous studies with galvanic vestibular stimulation and visual flow stimulation have shown that head-on-trunk position influences the direction of the resultant postural sway. A new model of vestibular-somatosensory interaction developed by Horak and Peterka have shown that sinusoidally oscillating a platform on which a subject stands increases the weighting of vestibular information for the formation of postural responses. We will use this model to investigate the interaction between cervical afferents and the vestibular system in control subjects, and subjects with unilateral vestibular lesions. In experiment 1 we will determine whether the lateral bias of postural verticality observed during sinusoidal surface oscillations with the head facing forward will change to a more anterior/ posterior bias when the head is rotated. In experiment 2 we will determine whether the lateral bias of verticality will be observed during ambulation with various head positions.